Union Square Ventures LLC, the investment firm that helped seed Twitter, Tumblr, Foursquare, Kickstarter and Etsy, is raising a new early-stage venture fund.

Massachusetts Pension Reserves Investment Board approved a $15 million commitment to the fund at a meeting Tuesday, said Eric Convey, the board’s director of communications.

“We look at them as a good early-stage investor,” he said in a phone interview. With the latest commitment, Massachusetts has pledged $87.5 million to various Union Square funds, he said.

Union Square’s last two funds were both of the 2014 vintage: the $174.4 million late-stage Union Square Venture Opportunity 2014 LP and the $166 million early-stage Union Square Ventures 2014 LP, according to data compiled by Bloomberg.

A 2012 early-stage fund, which is still investing the $116.5 million it raised, had earned a net internal rate of return of 48 percent, as of June 30, 2015, according to a filing from the Oregon Public Employees Retirement System. The firm’s $200 million late-stage, opportunities fund in 2011 had a net IRR of 68 percent at the same date, according to the document.

The 2014 funds have invested only a small portion of their capital, so their returns to date are not meaningful.

An official at Union Square declined to comment.

The firm manages $1 billion in six funds, according to its website.

In addition to the Massachusetts and Oregon pensions, other repeat investors in Union Square funds include University of Texas Investment Management, Los Angeles County Employees Retirement Association and Weyerhaeuser Co., according to data compiled by Bloomberg.

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